Thermal energy storage (TES) systems can store heat or cold to be used later under varying conditions such as temperature, place, time, or power. The main use of TES is to overcome the mismatch between energy generation and energy use [1]. The main requirements for the design of a TES system are high energy density in the storage material (storage capacity), good heat transfer between the HTF and the storage material, mechanical and chemical stability of the storage material, compatibility between the storage material and the container material, complete reversibility of a number of cycles, low thermal losses during the storage period, and easy control. Moreover, one design criteria could be the operation strategy, the maximum load needed, the nominal temperature and enthalpy drop, and the integration into the whole application system.
Already in 2011, Arce et al. [2] calculated the potential of load reduction (L), energy savings (E), and climate change mitigation (as CO2 emissions reduction – RCO2) of TES in buildings in the EU. The applications considered were seasonal solar thermal systems (L=25,287 MWth; E=46,150 GWhth; RCO2=12,517,676 tons), district and central heating systems (L=1,453,863 MWth; E=2,326,182 GWhth; RCO2=630,957,558 tons), solar short-term systems (L=416,180 MWth; E=319,269 GWhth; RCO2=86,599,153 tons), and passive cold systems (L=9,944 MWth; E=18,148 GWhth; Ee=6,481 GWhe; RCO2=3,085,135 tons).
Borehole used in BTES systems
There are three technologies of TES systems, each one with different performance, which will drive for which technology each one is more appropriate. Moreover, each technology is in a different maturity status. Sensible TES is when the energy is stored increasing or decreasing the temperature of a material (i.e., water, air, oil, bedrock, concrete, brick). Latent TES uses the phase transition, usually solid-liquid phase change, of a material (i.e., water turns into ice).; The materials used in latent TES are therefore called phase change material (PCM). The last technology includes sorption and chemical energy storage and is usually known as thermochemical TES.
Several review can be found in the literature on TES for building applications, such as PCM for heating and domestic hot water (DHW) [3], PCM for air conditioning [4], PCM in building envelopes [5], adsorption for cooling in buildings [6], TES in hybrid systems [7], TES for seasonal storage [8], or more general about the use of TES in building applications [9–11]. Moreover, TES systems also have an important role in district heating and cooling systems [12].
Water tank www.(istockphoto.com)
This factsheet describes sensible TES for short- and mid-term storage, while long-term is considered in the seasonal TES factsheet. Water energy storage has been used for many years. Today it is considered that TES in water tanks has an important role on the final efficiency of many energy systems, specially those including renewable energy [9,13]. In these storage tanks, attention should be given to ensuring the stratification of the water inside the tank, avoiding heat losses with good insulation of the tank and all auxiliaries, and the temperatures of operation. A way to increase the energy density of water tanks, also ensuring the water stratification, is the inclusion of PCM in the water tanks [14–16].
Another sensible TES technology is underground TES, including both aquifer (ATES) and borehole (BTES) [9,13]. The characteristics of these systems are:
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BTES (borehole TES): It is suitable for soils with rock or water saturated with no or only very low natural groundwater flow. It usually has 30 to100 m depth and a heat storage capacity of 15-30 kWh/m3. The heat is directly stored in the water saturated soil, and it injected in it with U-pipes.
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ATES (aquifer TES): It uses aquifers with high porosity, ground water, and high hydraulic conductivity, as well as small flow rate. It usually has a heat storage capacity of 30-40 kWh/m3.
IMPACTS:
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GHG emissions
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One study showed that an ATES system, including heat pumps, can reduce CO2 emissions up to 80%, compared to conventional gas-fired boilers and cooling machines [17]
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Energy consumption
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The same study showed that ATES system, including heat pumps, can reduce the primary energy consumption up to 75%, compared to conventional gas-fired boilers and cooling machines [17]
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Energy savings
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ATES systems can achieve 40-70% energy savings [18]
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Costs
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Hot and chilled water tanks cost are around 0.1-0.13 €/kWh [13,19]
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A BTES system can have a CAPEX cost about 15% higher than a conventional system [20]
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An ATES system, including heat pumps, can have a payback time between 4 and 8 years, compared to conventional gas-fired boilers and cooling machines [17]
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